1. Technical Field
The present invention relates to means for alerting certain fish and marine mammals that an underwater structure is nearby. More particularly, the present invention relates to "pingers" that transmit acoustic signals in a frequency range corresponding to the hearing capability of the species of interest. Still more particularly, the present invention relates to the watertight housing used to retain an electro-acoustic transducer. The housing is designed to provide sufficient acoustic output while minimizing power usage.
2. Description of the Prior Art
Widely used commercial fishing techniques involve the distribution of nets underwater which entangle targeted fish species for harvest. An unwanted result is that non-targeted fish and mammals may also become entangled in the nets. These "bycatch" species have an associated mortality which is desired to be minimized. Marine mammals in particular have shown the ability to avoid fishing nets if they are made aware of the nets. One means to alert the mammals to the presence of the net is the use of acoustic pingers along the length of the net. The characteristics of the pinger acoustic emissions are chosen so that the marine mammals are alerted, but the targeted fish species are not. It has been found that most commercially targeted fish have a hearing range that extends to no more than one or two kilohertz. Marine mammals have hearing ranges that extend up to several tens of kilohertz. Therefore pingers with acoustic outputs in the frequency range of a few kilohertz to several tens of kilohertz can perform the desired function of alerting the marine mammals to the presence of the fishing gear.
A pinger is basically a battery-powered acoustic transmitter which is attached to underwater fishing nets. The pinger emits acoustic waves which alert marine mammals to the presence of the net. This invention relates to the construction of the pinger to maximize the amplitude of the acoustic emission into the water. The pinger is enclosed in a watertight housing. A battery provides power to an electronic oscillator. The oscillator drives an electro-acoustic transducer which produces sound emissions. These sound emissions travel from the electro-acoustic transducer, through the medium inside the pinger (air), through the watertight housing and into the water.
Standard practice has been to use a watertight housing of minimum dimensions to contain the pinger components (battery, oscillator, and transducer). This results in minimized cost of the housing. However it may not provide maximum acoustic output per unit of battery power (or equivalently maximum battery lifetime at a given acoustic output level). Experimental pingers made using this construction have demonstrated operational battery lifetimes on the order of two weeks. This is not sufficient for widespread commercial acceptance.
The acoustic output is limited by the power available to drive the electro-acoustic transducer. Pingers are battery-powered devices. Because of the difficulty of changing the batteries and resealing the watertight housing, it is desirable to drive the transducer with as little average power as possible while still providing an adequate acoustic output to perform the warning function. Driving the transducer with as little average power as possible maximizes the battery lifetime.
In related prior art, a patent issued to Fuentes et al (U.S. Pat. No. 4,805,339) describes a fish lure with a sound transmitter and a means for increasing the sonic output into the water. In this case the means for increasing the sonic output is a fluid coupling of the sound transmitter to the lure housing. No effort was disclosed to create a resonant acoustic chamber. A patent issued to Zaccheo (U.S. Pat. No. 5,291,682) describes a fishing net system made species-selective by an underwater acoustic transmitter. No effort is disclosed to increase the sonic output. A patent issued to Parra (U.S. Pat. No. 5,134,592) describes an acoustic method for scaring dolphins away from tuna nets by transmitting acoustic recordings of killer whales. No acoustic enhancements are disclosed.